"The downgrades and Negative Outlooks reflect growing risks for Japan's sovereign credit profile as a result of high and rising public debt ratios," said Andrew Colquhoun, Head of Asia-Pacific Sovereigns at Fitch, in a statement. "The country's fiscal consolidation plan looks leisurely relative even to other fiscally-challenged high-income countries, and implementation is subject to political risk."

Reuters said the downgrade could serve as a chilling reminder to highly indebted countries in Europe that urgent action is needed to trim public debt and prevent concerns about sovereign debt from weighing further on the global economy.

Several euro zone countries have been hit with multiple downgrades as the region struggles to deal with its mounting debt crisis.

The United States is expected to reach a $16.4 trillion government debt ceiling after November presidential elections, laying the ground for another protracted political battle over how to cut the budget, similar to a bitter debate that rattled financial markets last year.